Known protocols for accessing a channel in the absence of a central entity (which are referred to more simply in the context of the present document as “multiple access protocols”) may be classified into two categories, depending on the degree of coordination between users.
In the first category of protocols, no coordination is set up between users; in particular users do not listen to the channel prior to transmitting in order to verify that the channel is free. It is attempted to avoid collision between the packets transmitted by different users by providing for each user to transmit packets at instants that are random.
A major drawback of such protocols is the inevitable existence of collisions between the packets transmitted by the different users, and thus inevitable loss of data, since there necessarily exists a non-zero probability of a packet being transmitted by one user before the end of the transmission of a packet by another user.
One solution to that drawback is proposed in the article by G. Liva, E. Paolini, and M. Chiani entitled “High-throughput random access via codes on graphs” (FUNEMS 2010). In a protocol belonging to said first category, it consists in introducing coding for correcting erasures so as to make it possible to recover the data lost in collisions. According to that article, each user transmits one or more replicas of each packet, such that when one of the replicas is received without collision, it is possible to subtract it from the analysis of collisions in which it is involved. A major drawback of that solution is that it operates at a level of the signal output by the demodulator (and thus on analog values), with all of the problems associated with imperfect estimation of channel gains (otherwise errors propagate during the iterations of the erasure correction method) and due to the presence of noise. Furthermore, the method described it that article is performed on geostationary satellite links, that can be modeled as a Gaussian channel characterized by gain and phase shift that are constant for the duration of a packet. That solution is difficult to envisage for multiple path channels of the kind observed with WiFi, i.e. for the 802.11 standard of the Institute of Electrical and Electronics Engineers (IEEE).
In the second category of protocols, each user listens to the channel, and transmits packets when no transmission is detected. A representative example of this second category is given by the distributed coordination function (DCF) of the media access control (MAC) layer in WiFi, which implements a protocol referred to as carrier sense multiple access/collision avoidance (CSMA/CA). In CSMA/CA, a first user seeking to transmit packets listens to the channel in order to determine whether or not another user is already sending packets; if so, the first user continues to listen to the channel; when the first user observes that the channel is free for a time interval that is constant and identical for all of the users, the user waits a little longer during a “backoff duration” prior to transiting a packet, said backoff duration having a value that is selected randomly in a time interval referred to as a “contention window”.
A first drawback of protocols in this second category is that collisions continue to occur therein, mainly for two reasons:                for each new backoff duration, the contention window is decremented; when it becomes zero, the terminal transmits its data without waiting, and thus with significant risk of collision, since it is not taking account of the real activity in the channel;        there exists a “hidden node” problem in which two terminals that cannot hear each other since they are too far apart transmit simultaneously over the same channel; nevertheless, in order to mitigate this “hidden node” problem, it is possible to use a mechanism known as request to send/clear to send (RTS/CTS) in which an access point transmits authorization to transmit to one of the terminals that has sent it an RTS, and the CTS sent by the access point is received by the other terminals that lie within the coverage of the access point, so that those other terminals put off their own transmissions until later.        
A second drawback of protocols in the second category is that listening to the channel prior to transmitting packets succeeds in reducing the number of collisions to which a user is subjected only at the cost of a non-negligible reduction in the time spent transmitting: specifically, users spend a great deal of time waiting to be sure that the channel is free. As a result, the aggregated throughput of data of such a system, which is equivalent to its spectral efficiency is generally sub-optimal.